In recent years, so as to keep down consumer energy demand and thus keep down emissions of CO2 gas which is feared to have an adverse effect on the global environment, the frequency of use of opening means such as windows having a higher thermal insulation performance and better energy saving than conventionally has increased, and such opening means have rapidly become widespread.
Furthermore, opening means having a thermal insulation performance comparable to that of walls and floors, specifically a thermal conductance of approximately 0.6 W/m2K (0.5 kcal/m2 hr° C.) or less, have gradually come to be demanded. The requirement of the thermal conductance of an opening means being not more than 0.6 W/m2K cannot be satisfied with hitherto widespread multi-layered glass panels in which dry air is filled between two glass plates, and hence research into opening means having novel structures is being carried out with vigor. With multi-layered glass panels, it is already commonplace to fill with a noble gas such as argon or krypton having a lower thermal conductivity than dry air instead of dry air, or use low-emission (low-E) glass in which a low-emission layer having a low-emission function of reflecting infrared radiation has been applied onto a surface of the glass plate that contacts the dry air, thus reducing the part of heat transmission due to radiation; however, opening means having a thermal conductance of not more than 0.6 W/m2K have not yet been realized.
One can thus envisage a multi-layered glass panel in which there are three or more glass plates and two or more layers filled with gas, and hence high thermal insulation performance is realized. With such an opening means, one assumes that a noble gas would be used as the gas, and low-E glass would be used for the glass plates; the thickness of the opening means would be high, and hence installation into a window frame or the like would be complicated, and moreover the weight would be high, and furthermore even slight optical absorption by the glass would be multiplied and hence the transmittivity would be reduced and thus the inherent light-transmitting ability would no longer be sufficiently displayed.
A thermally insulating vacuum glass panel in which a hollow layer formed between mutually facing glass plates is made to be at low pressure has been proposed as a multi-layered glass panel. However, atmospheric pressure always acts on the surface of each glass plate on the opposite side to the hollow layer side, and hence to counteract this and stably maintain the hollow layer, spacers must be disposed in the hollow layer. However, heat will flow through the spacers, and hence the excellent thermal insulation performance obtained through the low pressure will be marred by the presence of the spacers.
In addition to such a thermally insulating vacuum glass panel, other thermally insulating vacuum glass panels, and thermally insulating glass panels having a novel construction in which ordinary glass plates are disposed with a gap therebetween and the hollow layer thus formed is filled with dry air or a noble gas having a low thermal conductivity such as argon or krypton have also been proposed.
However, even with a combination of a conventional thermally insulating vacuum glass panel, a noble gas, and low-E glass, it has not been possible to realize a glass panel having a thermal conductance of not more than 0.6 W/m2K.
Thermally insulating vacuum glass panels enabling easy provision of light-transmitting opening means having high thermal insulation performance with a thermal conductance of approximately 0.6 W/m2K or less are awaited, and it is necessary to once again take a fresh look at the design of thermally insulating vacuum glass panels, and thoroughly eliminate heat transmission pathways.
The amount of conduction of heat through spacers depends on the density of arrangement of the spacers per unit area of the opening means, i.e. the spacing (pitch) of the spacers, the average area of contact with the glass plates per spacer, the thermal conductivity of the spacers, and the thermal conductivity of the glass plates. If the spacing of the spacers per unit area of the opening means is wide, and the average area of contact with the glass plates per spacer is small, then the thermal conductivity will be low and hence the thermal insulation ability of the opening means will be improved, but an excessive tensile stress may be produced at the surface of each glass plate on the atmosphere side opposite the surface of contact with the spacers, leading to natural breakage of the glass plate, and hence careful studies must be carried out (it goes without saying that the tensile stress produced is related not only to the average density of arrangement of the spacers but also to the Young's modulus of the glass plate and the thickness of the glass plate). Furthermore, compressive stress acts on the spacers due to atmospheric pressure, and hence it is necessary to make the spacers have sufficient compressive strength. A procedure for carrying out these studies is disclosed in Published Japanese Translation of PCT Application (Kohyo) No. H07-508967.
In Published Japanese Translation of PCT Application (Kohyo) No. H07-508967, the design is carried out under the assumption that ordinary annealed glass is used, and thus that there is no surface residual compressive stress. If the design procedure of Published Japanese Translation of PCT Application (Kohyo) No. H07-508967 is followed, then with glass plates of thickness 3 mm, 22 mm is the lower limit for the average spacing of the spacers, and at below this value, the probability of self destruction occurring from immediately above the contact surface of a glass plate with a spacer can no longer be ignored.
It is an object of the present invention to provide a light-transmitting glass panel having high thermal insulation performance with thermal conductance sufficiently reduced.